Voice radio — faster, simpler, more flexible — was the obvious tactical solution.

But voice radio was also the most easily intercepted form of communication.

Any receiver tuned to the right frequency could hear it.

And Japanese signals intelligence was, by the first years of the war, listening.

This was the problem that a civilian engineer named Philip Johnston brought to the Marine Corps’ attention in early 1942, along with a proposed solution that the Corps’ leadership initially regarded with skepticism and that would ultimately prove to be one of the most effective communications innovations in the history of warfare.

Philip Johnston’s Proposal